"Smart people want to give their best and as they learn more, they learn that they need to learn a lot more before they start sharing."
I'd be reluctant to share what I know about, say, website security, because I have this dim awareness that there are probably lots of things I'm missing, and anything I post would be flamed to death.
Maybe that's because this is "the knowledge at my current level" that I'm not ready to share yet.
Or maybe it's because, although I seem to be effective, I secretly fear that I'm incompetent.
"Better to stay silent and be thought a fool than to speak and remove all doubt."
Although it's probably one of the symptoms, it's not the most important one.
They don't share because of their EGOS.